1. Field of the Invention
The present invention relates to an endoscope system, in which a light guide conducts light for illuminating a subject under inspection from a light source toward the subject, and a method of controlling illumination light for the endoscope system.
2. Description of the Related Art
In the medical field, endoscopes are frequently used to acquire images of internal sites of test subjects for the purpose of diagnoses or surgeries. These endoscopes mostly use an image sensor that captures images of the subject through an inspection window at a distal end of a probing portion that may be inserted into the subject body. The endoscopic images acquired through the image sensor are displayed on a monitor. Many of recent endoscopes have a function to change the image magnification by changing the focal length of an imaging lens system that is located between the inspection window and the image sensor. The image magnification increases with increasing focal length, zooming in the target site and enlarging the image magnification on the monitor.
With this type of zoomable endoscope, the diagnostic inspection is generally done first in an ordinary inspection mode at a low image magnification (short focal length), to make the screening to search for and locate a suspected site of lesions. Thereafter the focal length is set longer or moved to the telephoto side and, at the same time, the distal end of the endoscope containing the imaging lens system is set closer to the located site for macro imaging or close-up inspection to inspect the target site in the enlarged view.
Such close-up inspection allows checking the condition of the target site in more detail. Especially, the condition of superficial blood vessels, such as presence of any heteromorphic vessels, is an important factor for diagnosis and follow-up examination. In order to facilitate inspection of superficial blood vessels and improve the accuracy of diagnosis, it is known using such illumination light that contains a large fraction of blue light component with shorter wavelengths. This is because hemoglobin contained in blood shows high absorbance to the blue light component. Also because light components of longer wavelengths will reach the deeper layer of the target site, if the illumination light contains a lot of long-wavelength components, the deeper layer will be illuminated so much that the contrast of the superficial blood vessels to other tissues and organs will be lowered undesirably.
In the endoscope system, the illumination light is usually transmitted from a light source through a light guide to the distal end of the probing portion and projected toward the target site. The light guide is made of a bundle of optical fibers. Due to the properties of the optical fibers, light transmitted through the light guide will attenuate with the length of the light guide. Particularly, light components of shorter wavelengths tend to have greater attenuation rates.
JPA 1997-66020 discloses an endoscope that uses two kinds of light guides; one uses optical fibers with a large aperture size and the other uses optical fibers with a small aperture size. In this known endoscope, the light guide consisting of large aperture optical fibers is used for wide-angle luminous intensity distribution of the illumination light, whereas the light guide consisting of small aperture optical fibers is used for narrow-angle luminous intensity distribution of the illumination light. Since fluxes of wide distribution and fluxes of narrow distribution are respectively emitted from these light guides, a single element projection lens may be used for either kind of light guide without the need for any complicated illumination lens system. In this prior art, the intensity of the narrow distribution fluxes may be increased to illuminate deeper into a tubular site, or the intensity of the narrow distribution fluxes may be lowered to obtain illumination light of wide and flat luminous intensity distribution, which is suitable for inspection of a planer site.
Large aperture optical fibers are advantageous in order to obtain a large light volume because the large aperture optical fibers let the illumination light from the light source enter at a high efficiency. However, because the light will reflect so much inside the large aperture optical fiber and the refraction factor of the large aperture optical fiber is high, the blue light component will attenuate drastically while the illumination light is being transmitted through the large aperture optical fibers. Therefore, illumination light will little contain the blue light component as projected from the large numerical aperture optical fibers, and is unsuitable for inspection of superficial blood vessels. By contrast, the light guide consisting of small numerical aperture optical fibers tends to reduce the volume of light so much that this type light guide is unsuitable for the ordinary inspection. In order to obtain a sufficient volume of illumination light, a high power light source is necessary, which is disadvantageous in view of space efficiency.